A solution of Na+ ions (as NaCl) is passed through a column containing Dowex-50W-X8 cation resin (for exchanging positively charged ions) in the form of spherical beads. The resin is based on a styrene/divinylbenzene copolymer that has been sulfonated with sulfuric acid; the “X8” denotes 8% divinylbenzene. This resin can adsorb Na+ ions from solution and return in exchange the same number of H+ ions, so that the solution maintains electrical neutrality. The process can be represented by:
(1)
where the subscripts L and R denote liquid and resin, respectively.
As the solution passes through the column, it becomes more dilute in Na+ and more concentrated in H+, a change that can be followed by sampling the H+ concentration in the exit stream with a pH electrode. Eventually the resin becomes saturated with Na+ ions and further exchange ceases. The ion exchange process is reversible, so that the resin can be regenerated by allowing it to come in contact with a strong acid (HCl).
The exchange of a Na+ ion for a H+ ion is not instantaneous, since it takes time for a Na+ ion to diffuse from the bulk of the liquid to the surface of the resin and then to some position inside the resin where it can exchange for a H+ ion. (And conversely for the H+ ion, which has to migrate back to the bulk solution.) The rate of exchange is expected to become slower as the Na+ ion concentration in the resin approaches saturation. The present investigation attempts to find a correlation for the rate at which this exchange process occurs.
The bed can be operated either in: (a) upflow, at a liquid velocity such that the particles are fluidized, or (b) downflow, in which case the bed is fixed. Under condition (a), the bed is often assumed to be well mixed, and the mathematical treatment is greatly simplified. In this experiment, investigation ion exchange operation will be carried out only in an upflow mode.